U.S. patent number 6,074,971 [Application Number 09/191,546] was granted by the patent office on 2000-06-13 for ceramic ferroelectric composite materials with enhanced electronic properties bsto-mg based compound-rare earth oxide.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Army. Invention is credited to Luna H. Chiu, Louise C. Sengupta, Somnath Sengupta, Steven Stowell, Jennifer Synowczynski.
United States Patent |
6,074,971 |
Chiu , et al. |
June 13, 2000 |
Ceramic ferroelectric composite materials with enhanced electronic
properties BSTO-Mg based compound-rare earth oxide
Abstract
Ceramic ferroelectric composite materials comprising barium
strontium titte/magnesium and oxygen-containing compound composite
further doped with rare earth (lanthanide) oxides. More
particularly, these inventive composites are comprised of
Ba.sub.1-x Sr.sub.x TiO.sub.3 /Mg--O based compound/rare earth
oxide composite, wherein x is greater than or equal to 0.0 but less
than or equal to 1.0, and wherein the weight ratio of BSTO to Mg
compound may range from 99.75-20 wt. % BSTO to 0.25-80 wt. % Mg
compound, and wherein said rare earth oxide additive comprises less
than about 10 mole percent of the composite. The rare earth oxides
of the composite include all oxides of the lanthanide series
elements including scandium and yttrium, as well as combinations
thereof. The magnesium-based compound may be selected from the
group consisting of MgO, MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3,
and MgCO.sub.3. This new class of composite materials has enhanced
electronic properties including: low dielectric constants;
substantially decreased electronic loss (low loss tangents);
increased tunability; increased temperature stability; decreased
sintering temperatures; and low curie temperatures. The electronic
properties of these new materials can be tailored for various
applications including phased array antenna systems, capacitors,
transmission wire, wireless communication, and pyroelectric
guidance devices.
Inventors: |
Chiu; Luna H. (Abingdon,
MD), Sengupta; Louise C. (Warwick, MD), Stowell;
Steven (Havre de Grace, MD), Sengupta; Somnath (Warwick,
MD), Synowczynski; Jennifer (Joppa, MD) |
Assignee: |
The United States of America as
represented by the Secretary of the Army (Washington,
DC)
|
Family
ID: |
22705924 |
Appl.
No.: |
09/191,546 |
Filed: |
November 13, 1998 |
Current U.S.
Class: |
501/139 |
Current CPC
Class: |
C04B
35/053 (20130101); C04B 35/465 (20130101) |
Current International
Class: |
C04B
35/03 (20060101); C04B 35/053 (20060101); C04B
35/462 (20060101); C04B 35/465 (20060101); C04B
035/468 (); C04B 035/47 () |
Field of
Search: |
;501/137,139 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Group; Karl
Attorney, Agent or Firm: Clohan, Jr.; Paul S. Biffoni; U.
John
Government Interests
GOVERNMENT INTEREST
The invention described herein may be manufactured, used and/or
licensed by or for the United States Government.
Claims
What is claimed is:
1. A ceramic ferroelectric composite material, comprising:
(a) barium strontium titanate, said barium strontium titanate
represented as Ba.sub.1-x Sr.sub.x TiO.sub.3, wherein x is greater
than 0.0 but less than 0.7;
(b) a compound containing both magnesium and oxygen; and
(c) a rare earth oxide or a combination of rare earth oxides;
wherein said barium strontium titanate, said magnesium containing
compound, and said rare earth oxide or combination thereof are
combined in said composite material.
2. The ceramic ferroelectric composite material of claim 1, wherein
said barium strontium titanate is Ba.sub.1-x Sr.sub.x TiO.sub.3 and
wherein x=0.35 to 0.45.
3. The ceramic ferroelectric composite material of claim 1, wherein
said magnesium containing compound is selected from the group
consisting of MgO, MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3, and
MgCO.sub.3.
4. The ceramic ferroelectric composite material of claim 1, wherein
said magnesium containing compound comprises MgO.
5. The ceramic ferroelectric composite material of claim 1, wherein
said rare earth oxide is selected from the group consisting of
scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide,
praseodymium oxide, neodymium oxide, promethium oxide, samarium
oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium
oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide,
lutetium oxide, and combinations thereof.
6. The ceramic ferroelectric composite material of claim 5, wherein
said rare earth oxide is selected from the group consisting of
lanthanum oxide, cerium oxide, and neodymium oxide.
7. The ceramic ferroelectric composite material of claim 1, wherein
the weight ratio of said barium strontium titanate to said
magnesium compound ranges from about 99.75-20 percent by weight
barium strontium titanate to about 0.25-80 percent by weight
magnesium compound; and wherein said rare earth oxide is added to
said barium strontium titanate and magnesium compound material at a
molar percentage of less than about 10 mole percent.
8. The ceramic ferroelectric composite material of claim 7, wherein
said magnesium compound is selected from the group consisting of
MgO, MgZrO.sub.3, MgZrSrTiO.sub.3, MgAl.sub.2 O.sub.4, MgTiO.sub.3,
and MgCO.sub.3.
9. The ceramic ferroelectric composite material of claim 8, wherein
said magnesium compound comprises MgO.
10. The ceramic ferroelectric composite material of claim 7,
wherein said rare earth oxide is selected from the group consisting
of scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide,
praseodymium oxide, neodymium oxide, promethium oxide, samarium
oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium
oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide,
lutetium oxide, and combinations thereof.
11. The ceramic ferroelectric composite material of claim 10,
wherein said rare earth oxide is selected from the group consisting
of lanthanum oxide, cerium oxide, and neodymium oxide.
12. The ceramic ferroelectric composite material of claim 7,
wherein said weight ratio of said barium strontium titanate to said
magnesium compound is approximately 40 percent by weight barium
strontium titanate to approximately 60 percent by weight magnesium
compound.
13. The ceramic ferroelectric composite material of claim 12,
wherein said magnesium compound comprises MgO.
14. The ceramic ferroelectric composite material of claim 12,
wherein said barium strontium titanate comprises Ba.sub.0.6
Sr.sub.0.4 TiO.sub.3.
15. The ceramic ferroelectric composite material of claim 12,
wherein said barium strontium titanate comprises Ba.sub.0.55
Sr.sub.0.45 TiO.sub.3.
16. The ceramic ferroelectric composite material of claim 12,
wherein said rare earth oxide is added to said material at a molar
percentage of about 0.5 mole percent.
17. The ceramic ferroelectric composite material of claim 12,
wherein said rare earth oxides are selected from the group
consisting of scandium oxide, yttrium oxide, lanthanum oxide,
cerium oxide, praseodymium oxide, neodymium oxide, promethium
oxide, samarium oxide, europium oxide, gadolinium oxide, terbium
oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium
oxide, ytterbium oxide, lutetium oxide, and combinations
thereof.
18. The ceramic ferroelectric composite material of claim 17,
wherein said rare earth oxide is selected from the group consisting
of lanthanum oxide, cerium oxide, and neodymium oxide.
19. The ceramic ferroelectric composite material of claim 1,
wherein said material has a room temperature dielectric constant of
from about 79 to about 166.
20. The ceramic ferroelectric composite material of claim 1,
wherein said material has a loss tangent of less than 0.0008 at a
frequency of 250 kHz.
21. The ceramic ferroelectric composite material of claim 1,
wherein said material has a loss tangent of less than 0.04 at a
frequency of 10 GHz.
22. The ceramic ferroelectric material of claim 1, wherein said
material has a curie temperature of less than about -50.degree.
C.
23. The ceramic ferroelectric material of claim 1, wherein said
material has a tunability of greater than 4.0 percent.
24. The ceramic ferroelectric material of claim 1, wherein said
material has a temperature stability of TCP.sub.ppm in the range of
150 to 2000 ppm .
Description
FIELD OF THE INVENTION
The present invention pertains generally to ceramic ferroelectric
composite materials having enhanced electronic properties. More
particularly, the present invention is directed to a ceramic
ferroelectric composite comprising barium strontium titanate,
Ba.sub.1-x Sr.sub.x TiO.sub.3 (BaTiO.sub.3 --SrTiO3; referred to
herein as BSTO), and compounds containing both magnesium and
oxygen, wherein said BSTO/Mg--O compound composite is further doped
with rare earth (lanthanide) oxides. In a preferred embodiment, the
magnesium-containing compound comprises magnesia (MgO), thus
forming the composite BSTO/MgO/rare earth oxide. The addition of
rare earth oxides to the BSTO/MgO composite creates a new class of
ferroelectric materials having improved electronic and microwave
properties which can be tailored for specific applications.
BACKGROUND OF THE INVENTION
There exists a need for the fabrication of ceramic materials having
improved electronic properties which may be adjusted for a
particular intended use. The present invention pertains to novel
ceramic ferroelectric composite materials for use, for example, in
low loss dielectric and ferroelectric applications such as
wave-guides in phased array antennas and dielectrics in
capacitors.
The ferroelectric materials are a replacement for the more
expensive current driven ferrites that are currently used in phased
array antennas. The present invention describes ferroelectric
materials which provide adequate phase shift and have improved
material properties which can be tailored for various applications.
These properties include: (a) lower dielectric constants; (b)
substantially decreased electronic loss, i.e., low loss tangents
(tan .delta.); (c) increased tunability; (d) increased temperature
stability; (e) decreased sintering temperature during manufacture;
and (f) low Curie temperatures.
Current attempts to use ferroelectric materials employ porous
ceramics whose properties are less than ideal for their intended
applications. Porous ceramics of the barium strontium titanate
type, Ba.sub.1-x Sr.sub.x TiO.sub.3, are commonly employed in
ceramic phase shifter antennas. However, these materials display
certain deficiencies due to manufacturing process difficulties and
expense, as well as in their overall electronic and microwave
properties. These deficiencies include electronic inhomogeneity,
structural weakness, difficult reproducibility and process control
during manufacture, and large loss tangents (tan .delta.).
Barium strontium titanate, Ba.sub.1-x Sr.sub.x TiO.sub.3
(BaTiO.sub.3 --SrTiO.sub.3), also referred to herein as BSTO, has
been known to be used for its high dielectric constant, ranging
from approximately 200 to 6,000, in various antenna applications.
This was set forth by Richard W. Babbitt et al. in their
publication, "Planar Microwave Electro-Optic Phase Shifters,"
Microwave Journal, Volume 35(6), June 1992. This publication
concluded that more research needs to be conducted in the materials
art to produce materials having more desirable electronic
properties.
To address this need, it was subsequently discovered that BSTO
could be combined with various metal oxides to produce
ferroelectric composites having different and improved properties
for particular applications. See, for example; U.S. Pat. No.
5,312,790 describing BSTO-alumina; U.S. Pat. No. 5,486,491
describing BSTO-zirconia; U.S. Pat. No. 5,635,433 describing
BSTO-ZnO; U.S. Pat. No. 5,635,434 describing BSTO-magnesium based
compounds incorporated by reference herein; and U.S. Pat. No.
5,427,988 describing BSTO-MgO composites and incorporated by
reference herein. Of these, the BSTO-MgO composite has proven
particularly important in that it possesses superior electronic
properties for use in antenna systems.
The present invention provides a new class of ceramic ferroelectric
composite materials with enhanced electronic properties comprising
Ba.sub.1-x Sr.sub.x TiO.sub.3 --Mg and O containing compound
composite, said composite being additionally doped with rare earth
(lanthanide) oxides. The magnesium and oxygen containing compound
is preferably MgO, but can also be selected from the group
consisting of MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3, and
MgCO.sub.3. The doping can be carried out either with individual
rare earth oxides or in combinations thereof. The rare earth oxides
encompassed in the present invention include oxides of scandium
(Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymuim (Nd), promethium (Pm), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Where
the rare earth element is represented by M, the oxides are
generally of the formula M.sub.2 O.sub.3, although cerium gives
cerium oxide CeO.sub.2. Moreover, it is intended that the rare
earth oxide additives of the present invention include all
oxidation states of the rare earth elements.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
new class of ceramic ferroelectric composite materials having
enhanced electronic properties suitable for, but not limited to,
use in phased
array antenna systems and capacitors.
It is a further object of the present invention to provide a new
class of materials comprising a barium strontium titanate-magnesium
and oxygen containing compound composite further doped with rare
earth oxides.
It is a further object of the present invention to provide a new
class of materials comprising barium strontium titanate-magnesium
oxide composite further doped with rare earth oxides.
It is a further object of the present invention to provide a new
class of materials having electronic properties that can be
tailored for specific applications such as for use in wave-guides
in phased array antenna systems, or as dielectrics in
capacitors.
It is a further object of the present invention to provide a new
class of materials having low dielectric constants, substantially
decreased electronic loss (low loss tangents), increased
tunability, increased temperature stability, decreased sintering
temperatures during manufacturing, and low Curie points.
It is a further object of the present invention to provide a new
class of materials useful in low loss dielectric and ferroelectric
applications such as, but not limited to, phased array antenna
systems, capacitors, transmission wire, wireless communications,
and pyroelectric guidance devices.
It is a further object of the present invention to provide a new
class of materials which are tunable with very low loss insertion
and which can be readily used in a wide range of frequencies, for
example, from about 100 kHz to about 77 GHz.
Other objects and advantages of the present invention will become
apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the
present invention provides a novel class of ceramic ferroelectric
materials having improved electronic properties, said materials
comprising Ba.sub.1-x Sr.sub.x TiO.sub.3 --Mg and oxygen containing
compounds doped with rare earth oxides, wherein x is greater than
or equal to 0.0 but less than or equal to 1.0, and wherein the
weight ratio of BSTO to Mg containing compound may range from
99.75-20 wt. percent BSTO to 0.25-80 wt. percent magnesium
compound, and wherein said rare earth oxide additive comprises up
to 10 mole percent of the composite. Preferably, x=0.35 to 0.45,
and for many applications the rare earth oxide is added in an
amount of about 0.5 mole percent. In addition, the
magnesium-containing compound may be selected from the group
consisting of MgO, MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3, and
MgCO.sub.3.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing dielectric constant versus temperature
for composites of 40 wt. percent BSTO (Ba.sub.0.6 Sr.sub.0.4
TiO.sub.3) and 60 wt. percent MgO with the addition of 0.5 mole
percent of various rare earth oxide dopants.
FIG. 2 is a graph showing dielectric constant versus temperature
for composites of 40 wt. percent BSTO (Ba.sub.0.55 Sr.sub.0.45
TiO.sub.3) and 60 wt. percent MgO with the addition of 0.5 mole
percent of various rare earth oxide dopants.
FIG. 3 is a graph showing measured density versus sintering
temperature for composites of 40 wt. percent BSTO (Ba.sub.0.6
Sr.sub.0.4 TiO.sub.3) and 60 wt. percent MgO with the addition of
0.5 mole percent of various rare earth oxide dopants.
FIG. 4 is a graph showing measured density versus sintering
temperature for composites of 40 wt. percent BSTO (Ba.sub.0.55
Sr.sub.0.45 TiO.sub.3) and 60 wt. percent MgO with the addition of
0.5 mole percent various rare earth dopants.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The subject matter of the present invention relates to a new class
of ceramic materials which have sought-after properties for
application in, for example, phased array antenna systems or
capacitors. These materials are superior to other currently
employed ferroelectric materials because they have enhanced
electronic properties which can be tailored for specific
applications.
When one considers the optimization of electronic properties for
ceramic materials, the following parameters must be taken into
consideration:
(1) Dielectric constant: Dielectric constant is related to the
energy storage in the material. In general, the dielectric constant
should be low, ranging from approximately 30 to 2,500. A lower
dielectric constant is preferable for wave-guides so that impedance
matching for the circuit is easier. This low dielectric constant
range does not decrease the phase shifting ability of the material
if a sufficient length of the material is used, as insertion loss
does not depend on the dielectric constant. Also, since the loss
tangent (tan .delta.) increases with increasing dielectric constant
for these ferroelectric materials, lower dielectric constant
materials tend to have lower loss tangents and, therefore, less
insertion loss.
(2) Low Insertion Loss: The loss tangent (intrinsic to the
material) is related to the power dissipation in a material, i.e.,
it is a measure of how a material serves to dissipate or absorb
incident energy (microwave) and therefore is most effective in
antenna devices when the loss tangent is in the range of 0.001 or
less. The low loss tangent serves to decrease the insertion loss
and hence increase the phase shifting per decibel of loss.
Generally, as the frequency of operation increases, the dielectric
loss tangent also increases. This restricts the microwave
designer's ability to develop efficient high frequency devices.
Extremely low loss materials (0.0007) can be used at millimeter
wave frequencies.
(3) Tunability: Tunability is a measure of how much the dielectric
constant changes with applied voltage and is defined as (dielectric
constant with no applied voltage)-(dielectric constant with an
applied voltage)/(dielectric constant with no applied voltage). For
simplicity, tunability can be represented as T
wherein,
X=dielectric constant with no applied voltage; and
Y=dielectric constant with an applied voltage.
The amount of phase shifting ability is directly related to
tunability, therefore, higher tunabilities are desired. The
tunability of a material under an electric field of 2.0 V/.mu.m can
range from 1% to 60% depending upon the materials employed.
Electronic tunabilities at a field of 2 volts/micron would range
from 4% to 50% for this new class of materials.
(4) Temperature Stability: The temperature stability of a material
can be measured by its TCP.sub.ppm which is defined as:
wherein,
.epsilon..sub.max =the maximum dielectric constant in the
temperature range of interest;
.epsilon..sub.ref =the dielectric constant at the reference
point;
T.sub.max =temperature of maximum dielectric constant; and
T.sub.ref =temperature of reference point.
TCP.sub.ppm then represents a percentage change equal to parts per
million. As the TCP is decreased the temperature stability of the
material increases. Temperature stability allows these materials to
be used in applications such as high dielectric substrates. In
addition, with increased temperature stability the material can be
used in applications where there is a wide range of operating
temperatures, thereby preventing the need for environmental
controls.
(5) Curie Temperature (T.sub.c): This is the temperature at which
the peak dielectric constant occurs for a material. It is also the
temperature at which the material changes state from paraelectric
to ferroelectric. For many applications, such as at high altitudes,
low curie points (below -50.degree. C.) are beneficial because the
material will not then change phase at the operating temperature,
thereby preventing the need for heating or protection
circuitry.
The materials within the scope of the present invention can be
tailored to fall within the optimum characteristics outlined above.
These novel materials have less loss (lower loss tangents, tan
.delta.) than BSTO-MgO composite material at both 250 kHz and 10
GHz frequencies. Lowering of insertion loss with these materials
will result in fewer difficulties for the application of
ferroelectrics into phased array antennas. Moreover, these
materials will be more attractive for applications in, but not
limited to, transmission wire, wireless communications, low powered
capacitors and pyroelectric guidance devices.
In addition, tunability of these materials remains high, i.e., well
within the requirements for application to phased array antenna
systems. In some specific cases, the tunability of the material has
been doubled due to the addition of the rare earth oxides. For
example, when 0.5 mole percent La.sub.2 O.sub.3 is added to a
composite containing 40 weight percent Ba.sub.0.6 Sr.sub.0.4
TiO.sub.3 and 60 weight percent MgO, the tunability increased by
almost 40% over the non-rare earth doped material.
Furthermore, in some cases the addition of the rare earth oxide
dopant to the BSTO-MgO material increased the temperature stability
of the material without any detriment to other electronic
properties. Additionally, another advantage provided by doping with
rare earth oxides is that in some composites the sintering
temperatures are 25.degree. C. below that of the composites without
the rare earth additive.
The novel materials of the present invention comprise Ba.sub.1-x
Sr.sub.x TiO.sub.3 -Mg and oxygen containing compound ferroelectric
composite material further doped with rare earth oxides, wherein x
is greater than or equal to 0.0 but less than or equal to 1.0, and
wherein the weight ratio of BSTO to Mg compound may range from
99.75-20 wt. percent BSTO to 0.25-80 wt. percent magnesium
compound, and wherein said rare earth oxide additive comprises up
to 10 mole percent of the composite. Preferably, x=0.35 to 0.45,
and for many applications the rare earth oxide is added in an
amount of about 0.5 mole percent. In addition, the magnesium-based
compound may be selected from the group consisting of MgO,
MgZrO.sub.3, MgZrSrTiO.sub.3, MgTiO.sub.3, and MgCO.sub.3.
The rare earth oxides encompassed in the present invention include
oxides of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce),
praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),
europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),
holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and
lutetium (Lu). Where the rare earth element is represented by M,
the oxides are generally of the formula M.sub.2 O.sub.3, although
cerium gives cerium oxide CeO.sub.2. Moreover, it is intended that
the rare earth oxide additives of the present invention include all
oxidation states of the rare earth elements. Thus, the inventive
materials may be represented generally as BSTO-Mg and O Compound-M
oxide. The magnesium-containing compound may be, for example, MgO,
so that the new composite comprises preferably BSTO-MgO-M oxide. In
addition, the doping may be carried out with combinations of rare
earth oxides as opposed to individually.
There are many methods for producing these bulk materials. One of
the basic methods begins by obtaining powders of barium titanate
(BaTiO.sub.3) and strontium titanate (SrTiO.sub.3). The powders are
then stoichiometrically mixed in a slurry of organic solvent, such
as ethanol, and ball milled in a conventional manner using grinding
media. This particular mixture is then air-dried and calcined at
approximately 200-300 degrees below the sintering temperature for
several hours. The resulting BSTO powder is then sieved and mixed
with Mg compound, for example MgO, and the rare earth oxide, for
example, CeO.sub.2, in the correct ratios and re-ball milled in an
organic solvent with a binder. The final mixture is then air-dried
and subsequently dry pressed to near net shape at about 7,000 psi.
The final samples are sintered in air at the correct temperatures.
Sintering schedules may be ascertained by those skilled in that art
using a dilatometer. After sintering, the sample can be machined
and electroded for usage and analysis. The manufacturing process
when using MgCO.sub.3 in lieu of MgO is the same as that described
above except that the starting materials are BaCO.sub.3,
SrCO.sub.3, TiO.sub.2, and MgCO.sub.3 mixed in water as
solvent.
Tables 1 and 2 set forth the electronic properties of various
BSTO-MgO-rare earth oxide ceramic ferroelectric composite
materials. These tables reflect data for composites made by the
foregoing method, wherein the rare earth dopants, MgO, and the BSTO
are mixed.
TABLE 1 ______________________________________ Electrical Property
Data for 40 weight % Ba.sub.0.6 Sr.sub.0.4 TiO.sub.3 60 weight %
MgO, plus 0.5 mole % of rare earth oxide Room Percent Curie Rare
Temperature Loss Tangent Tunability (%) Tempera- Earth Dielectric
(tan .delta.) At 2 ture Dopant Constant at 250 kHz volts/micron
(.degree.C.) ______________________________________ No rare earth
126.82 0.0008 9.23 -35 oxide CeO.sub.2 129.91 0.00048 8.22 -50
Dy.sub.2 O.sub.3 122.68 0.00045 6.48 -60 Er.sub.2 O.sub.3 123.85
0.00047 6.76 -60 La.sub.2 O.sub.3 165.58 0.0011 14.70 -70 Nd.sub.2
O.sub.3 99.589 0.00072 8.00 -60 Sm.sub.2 O.sub.3 104.10 0.00024*
6.58 -60 Yb.sub.2 O.sub.3 107.43 0.00034 5.94 -65
______________________________________ *dispersive sample
TABLE 2 ______________________________________ Electrical Property
Data for 40 weight % Ba.sub.0.55 Sr.sub.0.45 TiO.sub.3 60 weight %
MgO, plus 0.5 mole % of rare earth dopant Room Loss Loss Percent
Curie Rare Temperature Tangent Tangent Tunability Temper- Earth
Dielectric (tan .delta.) at (tan .delta.) at (%) at 2 ature Dopant
Constant 250 kHz 10 GHz volts/micron (.degree.C.)
______________________________________ None 110.59 0.0005
0.00832
6.57 -50 CeO.sub.2 100.00 0.0003 0.00694 5.50 -60 Dy.sub.2 O.sub.3
104.68 0.0004 0.01404 4.44 -70 Er.sub.2 O.sub.3 106.67 0.0004
0.01917 4.44 -75 La.sub.2 O.sub.3 79.00 0.0014 0.0115 7.85 -80
Nd.sub.2 O.sub.3 100.06 0.0008 0.006988 7.71 -75 Sm.sub.2 O.sub.3
100.45 0.0016 0.0358 5.25 -75 Yb.sub.2 O.sub.3 109.60 0.0004
0.010016 4.86 -80 ______________________________________
As evidenced by the data, the addition of rare earth oxides to
BSTO-MgO composite can improve the electronic properties of the
ferroelectric composite material. The dielectric constant stayed at
low values which is ideal for the application of insertion into
phased array antenna because these dielectrics should make
impedance matching easier. Here again, the low dielectric constants
do not decrease the phase shifting ability of the material if a
sufficient length of the material is used. Also, in the case of
using a second dopant such as CeO.sub.2, warpage during sintering
was minimal. Therefore, fabricating long lengths of this material
can be accomplished fairly easily.
Also, as can be seen from the tables, the loss tangent can be
lowered 30-50% from that of the undoped composite material by some
of the rare earth dopants at low mole percentages. More
specifically, the loss tangents for both low and microwave
frequencies can be significantly decreased without detriment to the
other electronic properties. For example, in the case of BSTO-MgO
composite with 0.5 mole % of CeO.sub.2 additive, the loss tangent
at 250 kHz decreased from 0.0008 to 0.00048 which is a 40%
reduction in loss tangent. In addition, for a BSTO-MgO composite
with 0.5 mole % CeO.sub.2 the loss tangent at 10 GHz is decreased
from 0.0083 to 0.0069. This is a significant decrease in the
microwave region.
Furthermore, the tunability of the materials is maintained and is
well within the specification for phased array antennas. In the
case of doping 40 wt. % Ba.sub.0.6 Sr.sub.0.4 TiO.sub.3 and 60 wt.
% MgO with 0.5 mole % La.sub.2 O.sub.3, the tunability is 38%
higher than that of the composite without the rare earth
additive.
The Curie temperature, sintering temperatures, and temperature
stability can all be improved by additions of specific rare earth
additives. For example, in the case of BSTO-MgO composite with 0.5
mole % Er.sub.2 O.sub.3 the sintering temperature is 30 degrees
lower than 1450.degree. C., which is the normal sintering
temperature of the composite without rare earth additions.
Temperature stability was improved by adding 10 mole % Er.sub.2
O.sub.3 to BSTO-MgO producing a material having a TCP=156, whereas
without the rare earth oxide the TCP=4661. In this case, the
additive dampened the dielectric constant at the curie temperature,
which improves temperature stability properties. The addition of
rare earth additives in some cases causes a decrease in the curie
temperature, broadening the applicability of these materials to,
for example, avionics where the operating temperatures can be as
low as -40.degree. C. For example, BSTO-MgO with La.sub.2 O.sub.3
additive has a Curie temperature of -70.degree. C., whereas without
the additive the curie temperature is -35.degree. C.
As shown in the FIGS. 1 and 2, the dielectric constant remains
within range over operating temperatures. FIGS. 3 and 4 show the
measured densities versus sintering temperatures for the various
composite materials, indicating that sintering temperatures are
lowered for some rare earth doped materials. This lowered sintering
temperature is, of course, useful during manufacturing.
While the particular embodiments of the present invention have been
shown and described, it will be obvious to those skilled in the art
that changes and modifications may be made without departing from
this invention. It is therefore intended that the claims appended
hereto include all such obvious modifications, changes, and
equivalents as fall within the true spirit and scope of this
invention.
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